1. Field of the Invention
Embodiments of the invention are directed to semiconductor devices, and, in particular, semiconductor devices providing current control and self shut down functions.
2. Description of the Related Art
Ignition devices for internal combustion engines of vehicles use a semiconductor device installing a power semiconductor element to control switching of the primary side current in the ignition coil. FIG. 2 shows a conventional construction example of an ignition semiconductor device for an internal combustion engine of a vehicle using an insulated gate bipolar transistor (in the following, an IGBT) as the power semiconductor element. The construction example of ignition device shown in FIG. 2 is similar to that of Applicants' Japanese Patent Application No. 2010-178317.
The ignition semiconductor device of FIG. 2 comprises an electronic control unit (ECU) 1 for engine control, an ignition semiconductor integrated circuit (an ignition IC) 2, an ignition coil 7, a voltage source 10, and an ignition plug 11.
The ignition IC 2 comprises an output IGBT 4 for ON/OFF (short circuiting or opening) controlling the primary current in the ignition coil and a current control circuit 3 for controlling the collector current in the output IGBT 4, which is the primary side current in the ignition coil 7.
The voltage source 10 supplies a constant voltage for example 14 V to one terminal of the primary winding 8 of the ignition coil 7 connected to the voltage source 10. The other terminal of the primary winding 8 is connected to the C terminal, which is a collector electrode terminal, of the ignition IC 2. The E terminal, which is an emitter electrode terminal, of the ignition IC 2 is connected to the ground potential terminal (GND) and the G terminal, which is a gate electrode terminal, is connected to the ECU 1.
Now, operation of the ignition semiconductor device shown in FIG. 2 will be described in the following. The ECU 1 delivers a signal for ON/OFF controlling the output IGBT 4 of the ignition IC 2 to the G terminal of the ignition IC 2. When a voltage of 5 V, for example, is delivered to the G terminal, the output IGBT 4 of the ignition IC 2 turns ON; and when a voltage of zero volts is delivered to the G terminal, the output IGBT 4 of the ignition IC 2 turns OFF.
Upon delivery of an ON signal from the ECU 1 to the G terminal, the output IGBT 4 of the ignition IC 2 turns ON and a collector current Ic starts to flow from the voltage source 10 through the primary winding 8 of the ignition coil 7 to the C terminal of the ignition IC 2. The current variation rate dl/dt of the current Ic is determined by the inductance value of the primary winding 8 and the voltage applied to the primary winding 8. When the current Ic reaches a certain value, for example 20 A, which is controlled by the current control circuit 3, the current Ic is held at this constant value.
Upon delivery of an OFF signal from the ECU 1 to the G terminal, the output IGBT 4 of the ignition IC 2 turns OFF and the current Ic abruptly decreases. This abrupt change in the current Ic causes rapid increase in the voltage between the terminals of the primary winding 8, which in turn raises the voltage between the terminals of the secondary winding 9 up to several tens of kilovolts for example 30 kV. This high voltage is added to the ignition plug 11. The ignition plug 11 is designed to discharge at applied voltages high than about 10 kV.
If the ON signal is delivered from the ECU 1 for a time duration longer than a predetermined time for example longer than 10 msec, or the temperature of the ignition IC 2 is higher than the prescribed value for example 180° C., the ignition device is in an abnormal condition that may cause damage such as burning of the ignition coil 7 or ignition IC 2. In such abnormal conditions, the self shut down signal generator 15 generates a self shunt down signal Vsd by means of a timer circuit and a temperature detection circuit and drives operation of the self shut down circuit 16 to interrupt the current Ic.
The rapid shut down of the current Ic utilizing the current control function and the self shunt down function generates oscillation in the current Ic, which would cause erroneous ignition of the ignition plug and damages in the engine. In order to cope with the problem of erroneous ignition due to the oscillation of the current Ic, Japanese Unexamined Patent Application Publication No. 2008-045514 discloses a technique to slowly decrease the current Ic by providing a soft shut off circuit and setting a slow damping time. Besides, Japanese Unexamined Patent Application Publication No. 2006-037822 discloses a technique to set a slow damping time by providing an integration circuit composed of a diode and a capacitor.
The ignition semiconductor device shown in FIG. 2 controls the output IGBT 4 of the ignition IC 2 by a gate voltage control circuit 19, which will be described later, and executes slow damping of the current Ic at a rate dl/dt of, for example, about −1 A/msec, in such a range that avoids erroneous ignition of the ignition plug 11.
The following describes the circuit construction of the current control circuit 3 of the ignition IC 2 shown in FIG. 2. The current control circuit 3 is driven by the voltage between the G terminal and the E terminal, and comprises a sense IGBT 5, a sense resistance 6, a gate resistance 12, a reference voltage circuit 13, level shift circuits 14a and 14b, a self shut down signal generator 15, a self shut down circuit 16, an operational amplifier 17, a metal oxide semiconductor field effect transistor (simply referred to as a MOS in the following) 18, and a gate voltage control circuit 19.
The sense IGBT 5 has a collector connected in common to the collector of the output IGBT 4, a gate controlled by the gate voltage control circuit 19, and an emitter connected in series to the sense resistance 6. The sense IGBT 5 and the sense resistance 6 comprise the sense voltage detection circuit, and convert the sense current flowing in the sense resistance 6 to a voltage and generate a source sense voltage Vs that is the voltage obtained by converting the current value proportional to the current Ic. The operational amplifier 17 controls the gate voltage of the MOS 18 so as to equalize the source sense voltage Vs to a voltage value that is preliminarily set in the reference voltage circuit 13, and thereby control the gate voltages of the output IGBT 4 and the sense IGBT 5 through the gate resistance 12 and the gate voltage control circuit 19 to control the current Ic to a specified current value.
FIG. 3 shows an example of circuit construction of the reference voltage circuit 13, which is composed of a bias circuit and a voltage dividing circuit. The bias circuit comprises a DepMOS (a depletion metal oxide semiconductor field effect transistor) 22 and a MOS 23 series connected with a common gate terminal. The voltage dividing circuit comprises a resistance 24 and a resistance 25. The reference voltage circuit 13 with this construction generates a source reference voltage Vr by dividing the voltage obtained by the bias circuit and the voltage dividing circuit in a predetermined ratio. The set voltage of this source reference voltage Vr is used for controlling the rated current of the current Ic.
FIG. 4 shows an example of circuit construction of the level shift circuits 14a and 14b. The level shift circuits 14a and 14b have the same construction as shown in FIG. 4. Each of the level shift circuit 14a and 14b is composed of a bias circuit comprising DepMOS 26 and MOS 27 connected in series with a common gate electrode, MOS 29 composing a current Miller circuit with the MOS 27, and DepMOS 28 connected to the MOS 29 in series. The level shift circuit 14a or 14b receives an input signal, the sense reference voltage Vr or the source sense voltage Vs, controlling the gate voltage of the DepMOS 28 and generates and delivers an output signal, a reference voltage Vref or a sense voltage Vsns that is level-shifted to a predetermined voltage value.
FIG. 5 shows an example of construction of the self shut down circuit 16. The self shunt down circuit 16 is composed of a bias circuit comprising a DepMOS 30 and a MOS 31 connected in series with a common gate electrode, a MOS 34 composing a current Miller circuit with the MOS 31, a MOS 33 series-connected to the MOS 34, an inverter 32, and a capacitor 35. The MOS 33 is ON-OFF controlled by the self shunt down signal Vsd generated by the self shut down signal generator 15, and in the ON state in the normal operation condition and in the OFF state in the abnormal condition. The ON resistance of the MOS 33 is set at a value sufficiently smaller than the ON resistance of the MOS 34. In this setting, the reference voltage Vref, which is obtained by level-shifting the source reference voltage Vr, is charged on the capacitor 35 during normal operation and delivered from the output terminal. In the abnormal condition, the reference voltage Vref charged in the capacitor 35 is discharged to the GND through the MOS 34, gradually decreasing the output voltage from Vref to zero volts.
The operational amplifier 17 detects a difference voltage between the reference voltage Vref and the sense voltage Vsns, the two voltages being level-shifted through the level shift circuit 14a and the level shift circuit 14b, respectively. The gate voltage of the MOS 18 is controlled according to the detection results. If the reference voltage Vref is greater than the sense voltage Vsns, the MOS 18 becomes the OFF state; If the reference voltage Vref is smaller than the sense voltage Vsns, the MOS 18 becomes the ON state. Thus, the gate voltage controls the ON resistance of the MOS 18.
The gate voltage control circuit 19 shown in FIG. 6 has a circuit construction described in Applicants' Japanese Patent Application No. 2010-178317, applied by the applicant of the present application. The gate voltage control circuit 19 is operated by the voltage that is determined by the gate resistance 12 connected to the G terminal and the ON-OFF state of the MOS 18 with respect to the potential at the E terminal. The gate voltage VGout of the output IGBT 4 is controlled by the voltage divided by a shunt resistance circuit consisting of a resistance 41 and a resistance 42. The gate voltage control circuit 19 comprises a variable resistance circuit composed of a resistance 38, a resistance 39, and a MOS 40. The MOS 40 is driven by a gate voltage that is a voltage divided by a shunt resistance circuit consisting of a resistance 36 and a resistance 37. Thus, the ON resistance of the MOS 40 is controlled to vary the gate voltage VGsns of the sense IGBT 5, which is a divided voltage from the variable resistance circuit.
The gate voltage control circuit 19 receives the detection result of the difference voltage between the reference voltage Vref and the sense voltage Vsns, and provides a voltage difference (an offset voltage) between the gate voltage VGout for the output IGBT 4 and the gate voltage VGsns for the sense IGBT 5. Thus, the gate voltage control circuit 19 functions to suppress collector current oscillation in the processes of current control and self shut down and prevent the ignition plug from erroneous ignition.
The following describes operation of the ignition semiconductor device shown in FIG. 2 with reference to FIGS. 9(A) and 9(B). FIG. 9(A) shows a case in which self shut down is conducted after the current Ic has reached the limiting current Ilim. When an ON signal for example at 5 V is delivered from the ECU 1, the sense voltage Vsns increases, the sense voltage Vsns being a voltage elevated from the source sense voltage Vs in the level shift circuit 14b. When the sense voltage Vsns reaches the reference voltage Vref at the time t1, the reference voltage Vref being a voltage elevated from the source sense voltage Vr of the reference voltage circuit 13 in the level shift circuit 14a, the MOS 18 turns ON and the gate voltage VGout of the output IGBT 4 drops to a certain lower value while the relation Vref=Vsns is held under control of the operational amplifier 17. When a self shut down signal Vsd is delivered from the self shut down signal generator 15 at the time t2, the reference voltage Vref through the self shut down circuit 16 gradually decreases and the VGout decreases so as to maintain the relation Vref=Vsns. When the VGout decreases to the threshold voltage Vth of the IGBT 4 for example 2 V at the time t3, the current Ic is completely shut down.
Although the sense voltage Vsns does not decrease lower than the voltage, for example 0.5 V, determined by the level shift circuit 14b, the reference voltage Vref continues to decrease down to approximately zero volts. As a result, the VGout becomes sufficiently smaller than the Vth to shut down the current Ic completely. The level shift circuits 14a and 14b are provided so as to hold the relation Vsns>Vref>0 even in the state of Ic=0.
Normal operation conditions are assumed in the ignition semiconductor device shown in FIG. 2 in which the set current control value Ilim=20 A, driving voltage Vb of the voltage source 10 Vb=14 V, and the load resistance RL of the sum of the resistance of the primary winding 8 and the wiring resistance RL=0.6Ω. A situation changed from these normal operation conditions is considered in which the driving voltage Vb has decreased to 12 V, for example, or the load resistance RL has increased to 0.7Ω, for example. Operation in this situation is illustrated in FIG. 9(B). When an ON signal for example at 5 V is delivered from the ECU 1 at the time t4, the sense voltage Vsns increases but not reaches the reference voltage Vref and ceases to increase at a certain voltage and remains at that voltage. In this time, the current Ic=(Vb−Vc)/RL, where Vc is a collector voltage. When a self shut down signal Vsd is delivered from the self shut down signal generator 15 at the time t5, the reference voltage Vref through the self shut down circuit 16 begins to gradually decrease. Immediately after the condition Vref=Vsns is reached at the time t6, the VGout abruptly drops to a certain lower value and then gradually decreases so as to hold the relation Vref=Vsns.
In the case the self shut down operation is conducted by means of a timer circuit provided in the self shut down signal generator 15, even if a self shut down signal Vsd is delivered correctly from the timer circuit, a delay time t6−t5 arises and elapses until start of decrease in the current Ic. Thus, a virtual timer period changes depending on operation conditions including the driving voltage Vb and the load resistance RL.
FIG. 8A shows dependence of the delay time on the driving voltage Vb and FIG. 8B shows dependence of the delay time on the load resistance RL under the conditions a diffusion potential Vbi of the output IGBT 4 Vbi=0.6 V, an ON resistance Ron=0.07Ω, and a damping speed dl/dt=−1 A/msec. An ignition semiconductor device needs to deal with wide range of operation conditions taking wide variety of applications into consideration, and the delay time t6−t5 is thus desired invariant in the self shut down operation. Nevertheless, the delay time may be elongated to twice the predetermined timer period of 10 msec, for example, under some operation conditions. In the case the self shut down operation is triggered by temperature rise due to self heating, a self shut down signal Vsd is formed by a temperature detection circuit in the self shut down signal generator 15 and delivered from the self shut down signal generator 15. However, a delay time t6−t5 elapses from delivery of the self shut down signal Vsd until the current Ic starts to decrease. During the delay time, the operation temperature continues to increase. Therefore, an operation mode is desired in which the current Ic decreases immediately after delivery of the self shut down signal Vsd.